rlown wrote:what about if you're 140lbs and carrying a 36lb pack?...
The same as a 176 lb person. You burn about the same calories whether the weight is on your back or inside your skin.
BTW if you are 140lbs naked on the scale, do not forget to add the weight of your clothes and shoes. The pack is not the only weight your body is carrying.
Many now recommend calculating calories per mile instead of per hour -- more accurate. Per hour does not factor in speed (you burn more calories the faster you hike, but then you are covering the miles in less time). Using per mile is independent of speed.
Factoring in slope is more problematic. If it is round trip then ignore slope. The extra effort involved in going uphill is mostly offset by the lesser effort needed going downhill. Not completely offset, but close enough for quick estimates.
Of course, the old adage says the best exercise to lose weight is pushaways at the dinner table.